In a survivalist situation it might be necessary to start fishing to find food to survive. However this can be challenging because if you are in the wild there are no real rules that can help you tell edible fish from those that are poisonous.
However if a fish is going to be poisonous it is most likely to be living in shallow water around reefs or lagoons. The most poisonous fish also have boxy or round bodies with hard shell-like skins that are covered with bony plates and spines. A mouth like a parrot, small gills or no belly fins might also mean that the fish is poisons.
Many fish that feed off tropical reefs are poisonous because they carry ciguatera which is a toxin that lives in the coral they feed from. Avoid fish taken from beaches with crushed coral bottoms.
Most poisonous fish also tend to live on the leeward side of an island. Usually there are patches of living corals mixed with open sea. This is usually the type of off-shore hang out of poison fish.
If the water seems discoloured do not try to catch and eat the fish swimming in it. This can indicated that toxic plankton has taken over the water.
Fish in the windward side of or in deep passages. It is the deep water fish that are usually not poisonous.
Even though they look a bit like snails and seem like that are delicious you should never eat any type of animal that lives in a cone shell. Toxins from these can paralyse you. You should never pick one up because if it stings you can suffer from weakness, loss of coordination, numbness and other severe problems.
If you are in the Caribbean or Florida and fishing for octopus you should avoid a creature called The Blue Ringed Octopus. This octopus bites and its venom contains tetrodotoxin that blocks sodium channels and causes respiratory failure. You can identify this octopus by the fact that it is normally a dark yellowish brown but when it is disturbed the octopus develops blue rings.
If you are in tropical waters at the other end of the world you should also avoid the stone fish. This is one of the most venomous of fish. It looks exactly a stone. It is perfectly camouflaged against a rock. If it stings you it is fatal.